Omnivorous dinosaurs

[Karen Casino <kcasino@comcast.net>]

Just to let you know, this is purely a matter of a layman's curiosity.

So yesterday, in the course of conversation my Dad asked me if there  
were any omnivorous dinosaurs. I faked some knowledge (Uh, I think  
the ostrich mimics? Probably not the Heterodontosaurs?), but wound up  
having to say that I didn't know of any specifically, and that on  
reflection the whole subject of omnivorous behavior seemed complex  
and elusive.

Are there any anatomical features that indicate omnivory (sic - what  
would this word be?), and if so, do any dinosaurs have them? Is there  
a distinction between omnivores that can eat a variety of foods and  
those that have to eat a variety of foods? What distinctions are made  
between opportunistic feeding (the grass-eating dogs and chicken- 
eating mules of the world) and genuinely omnivorous behavior?

Nothing quite like those simple questions that illuminate one's vast  
gulfs of ignorance...